Electrolysis of brine has been playing an important role as a material industry. However, energy consumption required for electrolysis is high. In Japan where an energy cost is high, therefore, saving energy used for electrolysis has been an important problem.
An ion exchange method which has been currently a mainstream obtains aqueous solution of sodium hydroxide, chlorine and hydrogen by electrolysis of brine (reference should be made to the equation (1) described below). While the theoretical decomposition voltage by the ion exchange method is about 2.19 volts, the operation is practically conducted at an actually-required voltage (hereinafter referred to as “actual voltage”) of about 3 volts, because of the ohmic potential loss, the overvoltage of an electrode, etc.:2NaCl+2H2O→Cl2+2NaOH+H2  (1)
By contrast, in order to attempt a significant energy saving, a combination method has been investigated in which an ion exchange method is combined with a method using a gas diffusion electrode as a cathode to reduce oxygen (reference should be made to the equation (2) described below) and such a combination-method is (hereinafter referred to as “oxygen cathode method”).2NaCl+½O2+H2O→Cl2+2NaOH  (2)
The oxygen cathode method can lower the theoretical decomposition voltage to 0.96 volts and can be operated at an actual voltage of about 2 volts, even including the other resistance components. While no hydrogen is generated, the energy saving of 30% or more can be expected.
As a method which is an improved oxygen cathode method, a method is disclosed in Patent Literatures 1 to 3, in which the gas diffusion electrode is in close contact with the ion exchange membrane, more specifically a method is disclosed therein in which a cathode chamber is configured as a cathode gas chamber. Since this method is composed of two chambers, that is, the anode chamber and the cathode chamber, it may be referred to as a two-chamber method, contrary to a three chamber method composed of the anode chamber, the cathode chamber and the gas chamber. In this method, the gas diffusion electrode is brought into contact with the ion exchange membrane, and an elastic material (cushion material) is packed into the cathode chamber so as to compress the gas diffusion electrode uniformly to the entire surface of the anode via the ion exchange membrane by using the repulsive force generated therein. Further, in order to hold and discharge the aqueous solution of sodium hydroxide more securely, there is a case where a hydrophilic liquid-penetrating material is put between the ion exchange membrane and the gas diffusion electrode. This two-chamber method is an improved method in that the voltage or electricity consumption can be reduced, because the inter-electrode distance is minimized compared with the conventional three-chamber method.
According to the two-chamber method, it is possible that the aqueous solution of sodium hydroxide can be held by the liquid-penetrating material (that is, a liquid retention layer described in paragraph [0025] of Patent Literature 3) and electrolysis can be conducted stably by interposing the hydrophilic liquid-penetrating material between the ion exchange membrane and the gas diffusion electrode. There has been, however, a problem that a minute amount of calcium ion transferred to the cathode by water penetrating through the ion exchange membrane (hereinafter referred to as “penetrating-water”) easily deposits on the surface of the cathode facing the ion exchange membrane, depending on the material or structure of the liquid-penetrating material of the method. A calcium ion originates from impurities remaining in brine. Such a phenomenon on the surface of the cathode facing the ion exchange membrane is not observed in a three-chamber method.
In the ion exchange membrane method, it is required that the concentration of calcium ion in the brine supplied into the anode chamber should be maintained in low concentration under the strict control of purification of the brine. As one of such methods of purification, a method for removing calcium ion, etc, has been known in which the purification by a chelate resin is added to a brine-purification process comprising a flocculation reactor, a setting tank, a sand filter and a micro filter. Even if, however, the purification by a chelate resin is carried out, it is hard to remove completely a calcium ion in the brine, and a calcium ion remains in the brine in approximately 10 ppb. Some of the remaining calcium ions move toward the cathode through the ion exchange membrane along with penetrating water, and reacts with the aqueous solution of sodium hydroxide of high concentration when they reach to the vicinity of the surface of the ion exchange membrane to produce calcium hydroxide which is deposited in the vicinity of the surface of the ion exchange membrane. In the case of electrolytic cell in which the hydrophilic liquid-penetrating material is put between the ion exchange membrane and the gas diffusion electrode, the flow of the aqueous solution of sodium hydroxide is decreased at the point with which the hydrophilic liquid-penetrating material is in contact; and the calcium ions moving through the ion exchange membrane hardly diffuse and are bonded with hydroxyl ions to be easily deposited on the surface of the ion exchange membrane.